Conventionally, silica-based planar light-wave circuits (PLC) have been developed for practical use, and therefore array waveguide gratings (AWG) and optical splitters have played a key role as key components supporting recent optical communication markets. Recently, optical functional devices such as wavelength tunable light sources hybrid-mounted on semiconductor optical amplifiers (SOA) and silica-based PLC have been developed to achieve an inexpensive small-size system on a single chip by mounting active elements and passive elements on the common PLC substrate. However, due to complex and advanced functions required for optical functional devices, it is necessary to increase the size of PLC elements and power consumption for driving PLC, which in turn causes limitations to functionality and performance achieved by conventional silica-based PLC. This expedites research and development for SOI (Silicon On Insulator) waveguides adapted to silicon fine processing technologies using thin silicon-wires and photonic crystals (PC), thus studying key components with small sizes, low power consumption, and low cost. In particular, it is possible to produce optical waveguides whose sizes are significantly reduced in comparison with the size of conventional PLC, by use of thin silicon-wires. It is possible to miniaturize SOI waveguides by use of micro-optical circuits, made of silicon core materials, with high relative refractive indexes to clad materials (SiO2, dielectric). Compared with conventional silica-based optical waveguides with relative refractive indexes about 5% and bend radiuses of about 500 μm, thin silicon-wire optical waveguides can achieve relative refractive indexes of 40% or more and bend radiuses reduced to several microns. Using silicon materials, it is possible to produce optical elements serving as electrically active elements by way of integrated circuit technologies; hence, silicon materials have superior properties which cannot be realized in silica materials. Recently, small-size optical switches which can operate with low power consumption have been developed using silicon waveguides. However, it is necessary to increase power consumption used to hold control and operation of optical switches as optical-circuit paths become complex since it is necessary to hold optical-circuit paths applied to optical switches for several days or several months.
Recently, various technologies have been developed with respect to optical functional devices and optical waveguides. Patent Literature Document 1 discloses “Method and Apparatus for Phase-Shifting an Optical Beam in a Semiconductor Substrate”, in which a plurality of floating charge modulated regions to shift phases of optical beams responsive to charge concentration is disposed along an optical path in a semiconductor substrate through which an optical beam is to be directed along an optical path. The optical function device includes a capacitor structure, used to accumulate charges in an optical waveguide, by which path switching is carried out using refraction variations due to variations of accumulated charges. This configuration does not need any power to hold charge storage; hence, it is possible to reduce standby power used to hold paths in an optical functional device. Patent Literature Document 2 discloses “Silicon Optical Waveguide Disposing MOS Capacitor on Waveguide”. Herein, an electric field may change a free-carrier concentration of the uppermost layer or lower layers of a silicon optical waveguide; electric-field variations may cause variations of refractive indexes; then, refractive-index variations may cause variations of optical modes propagating through the silicon optical waveguide. It is possible to control optical-mode propagation by controlling electric-field variations. Patent Literature Document 3 discloses “Optical-Electronic Field Effect Transistor”. The optical-electronic field effect transistor includes an optical waveguide below a gate electrode and a lower layer using a semiconductor layer with smaller refractive index than an active layer, thus confining light in an optical waveguide buried in an active layer. Variations of microwaves applied to a gate electrode may change a carrier concentration of an optical waveguide, thus changing a refractive index. It is possible to modulate the phase of an optical signal transmitting through an optical waveguide due to refractive-index variations of an optical waveguide. Patent Literature Document 4 discloses “Optical Modulator”. The optical modulator includes an optical waveguide having a high-mesa waveguide structure in which optical confinement is achieved by clamping a core layer with a clad layer having a small refractive index, thus achieving optical phase modulation depending on an electric voltage applied to an optical waveguide.